Earth Knowledge Portal is an impressive collection of data that is tied together using the Google Earth Plug-in. The best way to explain what it does is to view their intro video.
History Pin is a way for millions of people to come together, from across different generations, cultures and places, to share small glimpses of the past and to build up the huge story of human history.
History Buff is a neat website that is useful to teachers of US History
DebateGraph - making the best ideas and arguments on all sides of any contentious public issue freely available to all and continuously open to challenge and improvement by all.
Google Demo Slam: Epic Docs Animation
Google Apps - More resources for Google Apps
http://goo.gl/d387 - Google A to Z
Zoom it - ZoomIt is screen zoom and annotation tool for technical presentations that include application demonstrations
Social Studies and other web stuff....
Current Events
Wolfram Alpha Computational Engine
Many more here...
Groups interested in Social Studies